Need tank circuit info

[jbasi!]

Just how sensitive are the values of the parallel inductor and capacitor in a tank circuit to the efficiency of the transmitter frequency in a crystal controlled transmitter.

I use single transistor, crystal controlled transmitters in my Box turtle research, but don't know if I am getting the most from my design. Here is my radio page on my research web site. **broken link removed**

I've tried to use web based calculators to determine the value of the C in the tank circuit but don't know if I should trust them, and don't really have a way to test. I am centered around 150 mhz transmit frequency.

Any ideas that could help me?

 

[DerStrom8]

What do you mean, "the efficiency of the transmitter frequency"?

I think you may have your terminology confused. Anyway, the values of the inductor and capacitor determine the frequency, and if one changes at all it will change the frequency of the oscillation. Here is a formula you can use:

where f0 is the frequency, L is the inductance (in Henrys) and C is the capacitance (in Farads).

 

[cowboybob]

Welcome to ETO, jbasi!

I'm not sure I fully understand your question. I went to your site.

The most "efficient" tank circuit will be one that resonates at exactly the same frequency as the crystal.

That said, any of the on-line calculators (that I've ever used) will give you an accurate C value with a known L value at a particular frequency (in this case) determined by the crystal. It's simple math and there's no reason I know of not to trust the results the calculators provide.

If, however, you have no test equipment, proving any of the values you wish to confirm will be impossible.

<EDIT> (Beat me by a hair, Matt)

 

[MikeMl]

Also, suppose your calculator specifies that you need 35pF to resonate some inductor at your operating frequency. Be aware that the stray capacitance of the inductor to ground, the inductor winding-to-winding capacitance, the transistor's collector-to-base and/or collector-to-emitter junction-capacitances, and stray wiring capacitance account for probably half of the 35pF, so the actual capacitance required to resonate that tank is likely less than 15pF...

 

[JimB]

Without suitable measuring equipment you efforts to improve such a simple circuit are likely to be frustrating at best.

At 150MHz, the stray capacitance and inductance in the component leads and components will be a significant proportion of the effective inductance and capacitance of the resonant circuit.

Looking at your website, at one point you comment that you are using antennas which are 5 inches long, this is likely the biggest loss of efficiency in the system.
Also in the website there is a schematic showing a transmitter with a 12 inch antenna and a 12 inch counterpoise, this will be much better, but 12 inches is still an bit short for 150MHz.
If you could make the antenna and counterpoise 18 inches long, that would be about optimum for 150MHz. But I guess that may be a bit big for the turtle!
Make the antenna as long as possible, and use a counterpoise.
Adding a counterpoise to a transmitter with a short 5inch antenna will give an improvement, probably more than fiddling with L and C values.

What do the turtles think about this:

Alien Abduction !

JimB

 

[MrAl]

Hello there,

In addition to the good ideas already presented here and some laughs too, here is another note for you to think about.

For a given resonant frequency, the larger the inductor value the sharper the response. This means it cuts more harmonics. It also introduces more series resistance however so there is going to be a limit on how far you can take this. The resistance at these frequencies would be the AC resistance of the coil most of all (and this is not the same as the reactance of the coil).

The downside to a sharper response is it becomes harder to tune, becoming very fussy and drift could be much more bothersome. This means you may even want to go the other way: make it less sharp so it is easy to tune and keep on frequency. This would mean making the inductor value smaller and the cap value larger for a given resonant frequency.